One of the most recognizable portraits of William Shakespeare is a fake, experts say.

According to Britain's National Portrait Gallery, the image – commonly known as the "Flower portrait" – was actually painted in the 1800s, not while the Bard was alive.

The art experts who work at the gallery say they found traces of chrome yellow paint dating from about 1814 embedded deep in the picture.

Shakespeare died in 1616, and the date that appears on the portrait is 1609.

"We now think the portrait dates back to around 1818 to 1840, exactly the time when there was a resurgence of interest in Shakespeare's plays," Tarnya Cooper, the gallery's 16th century curator, told the Associated Press.

The picture shows the famous author, his face framed by a wide collar, looking out at an angle.

It has often been used as a cover for collections of his plays. It is called the Flower portrait because one of its owners, Desmond Flower, gave it to the Royal Shakespeare Company.

The discovery is the result of four months' worth of testing that involved X-rays, ultraviolet light, microphotography and paint samples.

The portrait is painted on top of a 16th century image of the madonna and child.

It resembles another portrait, the so-called Droeshout engraving, which accompanied the first folio publication of Shakespeare's plays in 1623.

"Some believed that it was the portrait which the engraver copied, but it now turns out that it is a copy of the engraving," Cooper said.

"There have always been questions about the authenticity of the painting," said David Howells, curator for the Royal Shakespeare Company.

"Now we know the truth, we can put the image in its proper context in the history of Shakespearean portraiture."

Two other images of Shakespeare, the Chandos Portrait and the Grafton Portrait, are also being studied as part of the investigation, which comes ahead of the National Portrait Gallery's 150th anniversary.